Method and apparatus for manufacturing of a calcium carbonate product, the product and its use

ABSTRACT

A method and apparatus for producing a calcium carbonate product formed of small &lt;100 μm sized separate calcium carbonate particles. Calcium hydroxide is fed through a disintegration and spraying apparatus ( 14 ), operating on the principle of a pin mill, into a gas which contains carbon dioxide for precipitating calcium carbonate particles and which is inside a precipitation reactor ( 10 ). The temperature in the precipitation reactor is maintained at &lt;65° C.

This application is a divisional of application Ser. No. 11/631,325, nowU.S. Pat. No. 8,012,445, which is the National Stage of InternationalApplication PCT/FI2005/000313, filed Jul. 5, 2005.

The present invention relates to a method and apparatus for producing acalcium carbonate product formed of small calcium carbonate particles.The present invention relates in particular to a method and apparatusfor producing precipitated calcium carbonate, whereby calcium hydroxideis fed as small drops and/or particles into a gas which contains carbondioxide and which is inside a precipitation reactor.

KNOWN TECHNOLOGY

Calcium carbonate is typically produced from a calcium hydroxide sludgeby precipitation using carbon dioxide. Traditionally, a batch process isused in production, in which process carbon dioxide gas is fed as finebubbles into the calcium hydroxide sludge until a proper carbonationlevel has been achieved. The dry matter content of the calcium hydroxidesludge must thus not be too high, so that the gas bubbles will be ableto enter into the sludge. For example, the American patent publicationU.S. Pat. No. 4,927,618 suggests that calcium hydroxide sludge with adry matter content of 7.68% be used. This production method requires along time for carbonation; it is mentioned in the first example of theUS patent that carbonation takes 31 minutes.

On the other hand, it has also earlier been suggested that calciumcarbonate be precipitated in continuously operating apparatuses. Forexample, it is suggested in the American patent publication U.S. Pat.No. 4,133,894, that calcium carbonate be precipitated in threesequential, high columns, in which calcium hydroxide sludge, with a dropsize of 1-2 mm, is sprayed, using spray jets, from the top of the columninto carbon dioxide gas streaming upwards from the bottom of the column.The produced calcium carbonate is collected from the bottom of the thirdcolumn as fine, <100 nm sized particles. In this case, the calciumhydroxide sludge must also be very diluted, i.e. its dry matter contentmust be low, approximately 0.1-10%. In the first column, only 5-15% ofthe calcium hydroxide will carbonate. The majority will carbonate in thesecond column.

In many applications of precipitated calcium carbonate (PCC) it would beadvantageous to be able to use very small calcium carbonate particles,<100 nm in size, which are uniform in quality and nearly uniform insize. These types of applications are needed, for example, in thepharmaceutical, cosmetics, and food industries. The paint, plastics,rubber, pigment and paper industries, including the techno-chemicalindustry, also find use for this type of calcium carbonate product.

PURPOSE OF THE INVENTION

The purpose of the present invention is to present an improved methodand apparatus for producing a calcium carbonate product formed mostly ofseparate, stabile, and very small, <100 nm sized calcium carbonateparticles.

The purpose is thus also to present an improved method and apparatus forproducing a calcium carbonate product with a large specific surfacearea.

The purpose is furthermore to present a method and apparatus, whichprecipitate calcium carbonate particles from calcium hydroxide rapidly,and which is less dependent on the particle size and/or dry mattercontent of the calcium hydroxide product used than previous methods havebeen.

The purpose is thus also to present a method and apparatus which make itpossible to maintain a reaction temperature which is lower than normalduring the carbonation stage, i.e. during the precipitation of calciumcarbonate.

The purpose is furthermore to present a continuously operating methodand apparatus which enable simultaneous feeding and use of variousadditives in the production of calcium carbonate.

The purpose is furthermore still to present a method and apparatus,whereby it is easy to accomplish complete or nearly complete carbonationof a mineral substance, typically calcium hydroxide.

THE INVENTION

In order to achieve the objectives presented above, a method and anapparatus according to the invention are characterized in that which ispresented in the characterizing parts of the independent patent claimspresented later in this patent application.

The present invention relates to a method and apparatus for producingprecipitated calcium carbonate with a small particle size, typically<100 nm, and thus with a large specific surface area, in a continuouslyoperating process. The calcium carbonate particles of the product aremostly separate, stabile, homogenous and uniform in size.

The method typically comprises

-   -   the continuous feed of calcium hydroxide (Ca(OH)₂) as fine drops        and/or particles into a gas which contains carbon dioxide and        which is inside a precipitation reactor, in order to carbonate        the calcium hydroxide, i.e. in order to produce precipitated        calcium carbonate (CaCO₃) in the precipitation reactor.

In a solution according to the invention, calcium hydroxide or othersuitable Ca⁺⁺ ion sources can be used as a reactive mineral substance,from which calcium carbonate is formed by using carbon dioxide.Typically, in a solution according to the invention, calcium hydroxideis fed into the precipitation reactor as a calcium hydroxide sludge,i.e. as calcium hydroxide dispersed in water, such as lime milk, but itcan also be fed in as a calcium hydroxide solution. The material isadvantageously fed into the reactor through a disintegration andspraying apparatus located in the reactor or in association with it.

The disintegration and spraying apparatus is typically a so-calledimpact mixer, by which very fine drops and/or particles are formed fromthe calcium hydroxide sludge or solution. The impact mixer typicallyoperates simultaneously as a precipitation reactor, or as a part of theprecipitation reactor; therefore it is also possible to bring calciumhydroxide advantageously in contact with carbon dioxide for initiatingcarbonation reactions in this reactor. Using the impact mixer, whichoperates on the principle of a pin mill, it is also possible to mixcalcium hydroxide sludge with a high dry-matter content into the carbondioxide gas.

The disintegration and spraying apparatus is typically fitted at the topsection of the precipitation reactor, but it can also be fitted inanother location of the precipitation reactor assembly which is suitablefor the feeding of calcium hydroxide.

In addition to the calcium hydroxide sludge, a gas containing carbondioxide which effects precipitation and which may be pure or nearly purecarbon dioxide, or combustion gas, or other suitable gas containing CO₂,is continuously fed into the precipitation reactor. The gas can be feddirectly into the precipitation reactor from its bottom section, fromthe sides or from the top using a separate gas feeding apparatus. Thegas containing carbon dioxide is advantageously fed into theprecipitation reactor through the top simultaneously with the calciumhydroxide. The gas containing carbon dioxide can be fed into theprecipitation reactor through the disintegration and spraying apparatus,in which case the carbonation reactions will already begin in thatapparatus. If desired, carbon dioxide gas can, however, be fed usinganother suitable gas feeding apparatus, and in some other location ofthe precipitation reactor. In order to maintain the material balance inthe precipitation reactor, the material which contains calcium carbonateis also continuously removed from the reactor.

It has now been established, that in order to produce the smallparticles desired, typically <100 nm in size, or the separate calciumcarbonate particles with the large specific surface area desired, it isadvantageous to arrange for precipitation to take place in a loweredreaction temperature, below 65° C., typically in 10-65° C., moretypically in 30-65° C., most typically in <40° C. The temperature in theprecipitation reactor can be maintained at the desired lower level inseveral different ways.

In a typical method and apparatus according to the invention, a lowtemperature is maintained in the precipitation reactor by cooling. Thereare several cooling methods which can be used one at a time or severalmethods simultaneously. The temperature in the precipitation reactor canthus be lowered

-   -   by feeding at least some of the calcium hydroxide or other        corresponding Ca⁺⁺ ion source into the precipitation reactor at        a lowered temperature,    -   by feeding at least some of the carbon dioxide into the        precipitation reactor at a lowered temperature, even as dry ice,    -   by means of cooling elements installed in the precipitation        reactor, such as a cooling sleeve installed in the precipitation        reactor,    -   by circulating the material which contains calcium carbonate,        and/or the gas which contains carbon dioxide, from the        precipitation reactor into a cooler equipped with a heat        exchanger, and then circulating it back to the precipitation        reactor from the cooler,    -   by directing the material which contains calcium carbonate,        and/or the gas which contains carbon dioxide, from the first        precipitation reactor through a cooler into the second        precipitation reactor and/or    -   by using another suitable method.

The calcium carbonate material circulated into the cooler from theprecipitation reactor can be returned to the same precipitation reactor,or to the next precipitation reactor, through the disintegration andspraying apparatus located therein.

It has now been established, that by regulating the temperature of thecalcium hydroxide fed into the precipitation reactor, it is possible toessentially influence the particle size of the calcium carbonate productto be formed. The lower the temperature of the fed calcium hydroxide,the smaller the particles that will be produced. If a small particlesize is desired, it is advantageous to feed at least some of the calciumhydroxide, advantageously most of it, into the precipitation reactor atthe temperature of <30° C., typically 5-30° C., advantageously 10-20°C., most typically <17° C. When considering technical-economicalaspects, it is generally not possible to use very low initialtemperatures. Moreover, when a calcium hydroxide sludge is cooled duringcarbonation reactions, it is possible to obtain even smaller calciumcarbonate particles.

Using a solution according to the invention, it is possible to produce acalcium carbonate product, in which the size of the calcium carbonateparticles, d₅₀, is <100 nm, typically <70 nm, advantageously <40 nm. Thespecific surface area of the calcium carbonate product, consisting ofmostly separate calcium carbonate particles, is >20 m²/g, typically >40m²/g, advantageously >60 m²/g.

Calcium hydroxide is advantageously fed directly into the precipitationreactor, or through the disintegration and spraying apparatus fitted tothe reactor. In the disintegration and spraying apparatus, whichoperates advantageously on the principle of a pin mill (i.e. it iseither a so-called impact mixer or through-flow mixer), the material tobe fed into the precipitation reactor is a target of strong impacts ordouble impacts from high speed rotors equipped with pins, blades orcorresponding elements, which disintegrate and spray the materialtravelling through the apparatus very efficiently. The speed differenceof the rings of the adjacent rotors or adjacent rotors and stators is5-400 m/s, typically 5-200 m/s.

The dwell time of the calcium hydroxide, or other Ca⁺⁺ ion source whichis fed into the precipitation reactor, inside the disintegration andspraying apparatus, which operates on the principle of a pin mill, is<10 seconds, typically <2 seconds, and most typically <1 second.

An effective carbonation time, i.e. the time during which the calciumhydroxide, or other corresponding Ca⁺⁺ ion source, is in effectivecontact with the carbon dioxide gas in the form of a mist and/or drops,is very short in each precipitation reactor according to the invention,generally <1 min, typically <30 seconds, and most typically <10 seconds.A total effective carbonation time is correspondingly longer if thereare several sequential precipitation reactors, or if the material iscirculated several times through the same precipitation reactor.Efficient and fast disintegration and spraying of the calcium hydroxidesludge or solution, and rapid mixing of the mist into the carbon dioxidegas, enables a very short effective carbonation time, and thus a veryshort production time for the calcium carbonate product.

A method and apparatus according to the invention may comprisemulti-staged carbonation, i.e. a precipitation process in which

-   -   in the first phase of the process, typically the main phase,        calcium carbonate is precipitated from calcium hydroxide in the        first precipitation reactor;    -   the calcium carbonate precipitated in the first precipitation        reactor, and the remaining calcium hydroxide, is directed into        the second precipitation reactor;    -   in the second phase of the process, calcium carbonate is        precipitated from the second portion, typically the remaining        portion, of the calcium hydroxide in the second precipitation        reactor, and    -   the calcium carbonate precipitated in the second precipitation        reactor and the calcium carbonate directed to the second        precipitation reactor from the first precipitation reactor, as        well as any eventual remaining calcium hydroxide which was fed        into the precipitation reactor, is directed into the third        precipitation reactor, or if the calcium hydroxide has been        completely used up, the calcium carbonate is discharged from the        precipitation process.

A method and apparatus according to the invention enable complete ornearly complete carbonation of calcium hydroxide, so that stabile,separate and, on average, homogenously sized particles are formed. Thecalcium carbonate product is thus mostly uniform in quality. Essentiallyno agglomeration takes place in the product, thus the product does notessentially change even over a long period of time.

Now it has been established that the calcium carbonate particles to beformed in a precipitation reactor according to the invention can be keptseparate, and their particle size can be controlled, for example,

-   -   by regulating the temperature of the precipitation,    -   by regulating the temperature of raw materials,    -   by regulating the amount of the fed calcium hydroxide sludge or        solution and/or its dry matter content,    -   by regulating the amount of carbon dioxide gas    -   by regulating the revolution speed of the rotors, the rotor        structure, the number of rings and blades, and the blade        position of the disintegration and spraying apparatus, and/or    -   by using a suitable additive.

By regulating the dry matter content of the calcium hydroxide sludge orthe concentration of the calcium hydroxide solution, it is possible toinfluence the dry matter content of the calcium carbonate to be formedin the precipitation reactor. The dry matter content of the precipitatedcalcium carbonate is typically regulated to <30%, more typically to10-20%.

Characteristics of the calcium carbonate to be produced, such asparticle size, crystal shape, specific surface area, separateness and/oruniformity, can also be affected by using various additives. In somecases, it is possible to reduce cooling of the precipitation reactorwith the use of additives. The additives may also be used in order toobtain yet a smaller particle size.

Thus, in addition to calcium hydroxide and carbon dioxide, some polyol,for example sorbitol, can be fed into the precipitation reactor. Polyolcan be added

-   -   to the calcium hydroxide sludge to be fed into the precipitation        reactor, or to the extinguishing water used for producing this        sludge,    -   directly into the precipitation reactor, for example, into the        disintegration and spraying apparatus and/or    -   to the material containing calcium carbonate which is discharged        from the precipitation reactor.

Polyol, such as sorbitol, enable the formation of small calciumcarbonate particles, and it also affects their surface-chemicalproperties. The addition of polyol also enables the formation of verysmall particles with less cooling, possibly without any cooling at all.Polyol is typically added in the percentage of 0.1-3%, more typically1-2% of the product.

The characteristics of a calcium carbonate product produced according tothe invention can also be affected by using various additives, such asfatty acid compounds, typically stearin acid or resin acid. In addition,other additives, such as dispersing agents, can be used.

Now it has been recognized, that by feeding a reactive mineralsubstance, such as lime milk, according to the invention, in the form ofa very fine mist into carbon dioxide gas, which effects precipitation,the reactive mineral substance and the gas effecting the precipitationcan be caused to mix with each other remarkably easily, and veryefficiently for the precipitation of calcium carbonate. It is evenpossible to obtain complete carbonation in a short reaction time period,i.e. quickly.

The precipitation of calcium carbonate (PCC) from calcium hydroxidestarts immediately and the reactions take place remarkably quicklybetween the calcium hydroxide and the carbon dioxide. By regulating thetemperature of the fed mineral substance, the reaction temperature, thetemperature or the consistency of the fed gas, by using a method and anapparatus according to the invention, characteristics such as thespecific surface area and the particle size of the calcium carbonatebeing formed can be controlled.

It is assumed that the reactions occur faster and more efficiently themore finely that the mineral substance can be dispersed, i.e. the morefinely that it is fragmented when feeding it into the precipitationreactor. When using a calcium hydroxide solution, the material can bedisintegrated into especially small drops. The efficiency of sprayingcan also be affected by the constructions of the disintegration andspraying apparatus and the precipitation reactor.

According to the invention, the temperature during precipitation canthus be adjusted, lowered, so that the formed calcium carbonateparticles will stay separated, that is, they will not have anyparticular tendency to adhere to each other. The same result, separatedcalcium carbonate particles, is also achieved by using additives. Theaddition, for example, of polyols, such as sorbitol, will reduce thetendency of the particles to adhere to each other.

Using a disintegration and spraying apparatus which operates on theprinciple of a pin mill, such as an impact mixer or through-flow mixer,it is possible to feed a mineral substance, i.e. calcium hydroxide, anda gas which effects precipitation continuously and simultaneously intothe precipitation reactor. The mineral substance solution will bedispersed into the precipitating gas as very fine drops or particleswhich form a mist-like gas suspension, in which the gas and the reactivemineral substance used for precipitation are activated and veryefficiently mixed together. Using a solution according to the invention,the substances participating in the precipitation process arehomogenized as a gas suspension, in which the reactions betweendifferent components can take place instantly.

Using an apparatus which operates on the principle of a pin mill, thematerial being fed into the precipitation reactor can be directed intothe gas inside of the precipitation reactor which contains carbondioxide by sequential, repetitive impacts, double impacts, shear forces,turbulence, over and under pressure pulses, or other correspondingforces which disintegrate the mineral substance into very small, <200 μmparticles and fine mist.

An apparatus which operates on the principle of a pin mill comprisesseveral, typically 3-8, most typically 4-6, coaxial rings equipped withblades or the like, of which at least every other ring operates as arotor, and the adjacent rings as stators, or as rotors rotating inopposite directions or in the same directions at different speeds. Thering speeds of the rotors may be 5-250 m/s. The speed difference betweenthe adjacent rotors may be 5-400 m/s, typically 5-200 m/s. Mills ormixers operating according to this principle have been presented earlierin Finnish patent publications 105699 B, 105112B and in WO publication96/18454.

In an apparatus which operates on the principle of a pin mill, themineral substance is typically guided with the help of the rotors andeventual stators to move radially outwards. The extension of the rotorand eventual stator rings from the rings' centre outwards produces adifference in pressure between the inlet, i.e. the centre, and theoutlet, i.e. the outer ring in the through-flow mixer. The pressuredecreases outwards from the centre. The created pressure differenceassists in conveying the mineral substance through the apparatus. Theblades, or the like, fitted on the rings of the apparatus can targetboth impacts and double impacts to the outward-flowing material andcreate shear forces, turbulence and under and overpressure pulses, whichgrind, or disintegrate and spray the material. An apparatus whichoperates on the principle of a pin mill is able to efficiently handlemineral flows of both high and very low dry matter contents to suitprecipitation. Operation of the apparatus is easy to regulate. Thus, ina precipitation reactor according to the invention, it is possible toprecipitate mineral substances of different dry matter contents, such as<30%, typically 10-25%.

A method and apparatus according to the invention make it possible tofreely select the conditions, such as the raw materials, the feedingproportions of the raw materials, the pH, the pressure and thetemperature, that are best suited for each process. A solution accordingto the invention does not set any limits for these conditions.

In addition to the reactive material used for precipitation, typicallycalcium hydroxide, other substances, for example ones suitable for thefurther processing of the precipitated calcium carbonate, can be fedinto the precipitation reactor.

Additives suited for the surface treatment of particles can be added tothe mineral substance before it enters the precipitation reactor, whileit is in the precipitation reactor or after it exits the precipitationreactor; for example, additives which affect the hydrophobation ofsurfaces, particle growth or the capacity of the particles to remainseparate from one another. Typical additives include polyols, such assorbitol, sugar, fatty acids, such as stearin acid, resin acid,phosphoric acid, dispersing substances, such as water solutions ofsodium and ammonium salts of acrylic polymers. Additives to be used orsome of them can be fed into the precipitation reactor simultaneously.

The invention shall now be described in more detail with reference tothe attached figures, in which

FIG. 1 illustrates schematically and as an example, a verticalcross-section of a precipitation reactor according to the invention;

FIG. 2 illustrates schematically and as an example, a horizontalcross-section of a disintegration and spraying apparatus fitted into theprecipitation reactor presented in FIG. 1;

FIG. 3 illustrates schematically and as an example, a verticalcross-section of a second precipitation reactor according to theinvention;

FIG. 4 illustrates schematically and as an example, a horizontalcross-section of the disintegration and spraying apparatus for the typeof precipitation reactor presented in FIG. 3;

FIG. 5 illustrates schematically and as an example, a verticalcross-section of a precipitation reactor group according to theinvention;

FIG. 6 illustrates schematically and as an example, a verticalcross-section of a second precipitation reactor group according to theinvention;

FIG. 7 illustrates schematically and as an example, a verticalcross-section of a third precipitation reactor group according to theinvention.

FIG. 1 illustrates a continuously operating precipitation reactor 10according to the invention, comprising a precipitation vessel 12, adisintegration and spraying apparatus 14 fitted in the precipitationvessel, a feed pipe 16 for the calcium hydroxide sludge, a feed pipe 18for the precipitation-effecting carbon dioxide gas, and a discharge pipe20 for the calcium carbonate suspension. The apparatus additionallyconsists of an actuator 22, including the bearing and sealing assembly24 between the actuator 22 and the apparatus 14.

The material containing calcium hydroxide, the calcium hydroxide sludge,which is to be fed into the precipitation reactor, can be cooledaccording to the invention before being fed into the precipitationreactor. For example, a cooler 11 which will cool the sludge to thedesired temperature can be fitted into the feed pipe 16 for the calciumhydroxide sludge.

Similarly, the carbon dioxide gas can be cooled as desired using acooler 11′ before it is fed into the disintegration and sprayingapparatus 14.

The precipitation reactor 10 can be equipped with a cooling sleeve 13,as illustrated in FIG. 1, according to the invention, which can surroundnearly the whole apparatus, as illustrated in the figure, or only partof it. The cooling sleeve 13 is equipped with some conventional coolingmethod which is not presented here in more detail.

Additionally, or alternatively, one or several separate coolers 15,which are in contact with the material at the top of the reactor, and/ora cooler 15′, which is in contact with the calcium carbonate material atthe bottom of the reactor, can be fitted to the precipitation reactor.

Additionally, or alternatively, circulation of the material containingcalcium carbonate can be arranged in the precipitation reactor from thedischarge pipe 20, through a pipe 16′, to the feed pipe 16 which leadsto the disintegration and spraying apparatus 14. The pipe 16′ isequipped with a cooler 17. The material can be returned directly to theprecipitation vessel if desired, for example, to its bottom. Thecirculating material is cooled in the cooler 17 and is returned as acooling agent into the precipitation reactor.

A disintegration and spraying apparatus 14 fitted in the precipitationreactor, a horizontal cross-section of which is presented in FIG. 2, isan impact mixer or through-flow mixer operating on the principle of apin mill, which consists of 6 coaxially arranged rings 26, 26′, 26″, 28,28′, 28″ equipped with blades 26 a, 26′a, 26″a, 28 a, 28′a, 28″a. In theapparatus 14, the calcium hydroxide sludge to be fed into theprecipitation reactor, and other eventual solid substance, isdisintegrated into small fragments, liquid drops and/or solid particles,and is fed from the apparatus 14 as a mist-like material into theprecipitation vessel 12. The dwell time in the disintegration andspraying apparatus is very short, <10 seconds, typically <2 seconds,most typically even less than 1 second.

As the arrows presented in FIG. 2 indicate, the first rings 26, 26′, 26″of the disintegration and spraying apparatus 14 operate as rotors which,in the case presented in the figure, rotate counter-clockwise. Thesecond rings 28, 28′, 28″ also, adjacent to the first rings, operate asrotors, which, however, in the case presented in the figure, rotateclockwise. The speed difference between the rings of the rotors rotatingin opposite directions is 5-400 m/s, typically 5-200 m/s. Blades 26 a,26 a′, 26 a″ and 28 a, 28 a′, 28 a″, which are mounted on the rings,encounter the calcium hydroxide sludge travelling through the apparatusradially outwards, making it a target of repeated impacts and doubleimpacts. Simultaneously, as the blades approach each other, overpressureis generated between the blades of the adjacent rotors, andunderpressure is generated when the blades draw apart from each other.The pressure differences very quickly generate over and underpressurepulses in the sludge. Moreover, at the same time, shear forces andturbulence are also generated in the material travelling through theapparatus 14.

Calcium hydroxide sludge, and other eventual substance, is fed throughthe pipe 16, as presented in FIG. 1, to the centre section 30 of thedisintegration and spraying apparatus 14, from which the sludge travelsradially outwards towards the open outer edge 32 of the outermost ring28″ by the effect of the rotor blades and by the pressure differencecreated between the centre and the outer ring of the apparatus. Thecalcium hydroxide sludge, and other eventual substance, can also be fedinto the apparatus 14 between the rings if desired. The calciumhydroxide and other eventual substances can be fed in to the apparatus14 through separate pipes if desired, in which case they do not come incontact with one another until they are in this apparatus.

Impacts and double impacts, shear forces, turbulence and under andoverpressure pulses generated by the rotor blades, rotating in oppositedirections, disintegrate the calcium hydroxide sludge to very finefragments, liquid drops and solid particles. In a solution according tothe invention, however, the material is able to flow a relatively openroute through the rings, and thus there is no risk of blocking in theapparatus.

In the solution according to the invention presented in FIGS. 1 and 2,the gas effecting the precipitation, CO₂, is directed through a pipe 18to the centre section 30 of the rings of the disintegration and sprayingapparatus 14. Additionally, or alternatively, the gas effecting theprecipitation can be fed between the rings if desired. From this centrelocation 30, or from the space between the rings, the gas flows radiallyoutwards, generating, in both the apparatus 14 and in the precipitationvessel 12 around it, a gas space 34 containing the gas which effects theprecipitation. The gas is discharged through a pipe 21, located on thetop section of the precipitation reactor. Some of the discharged gas canbe circulated back to the precipitation reactor through a pipe 18′. Thepipe 18′ is equipped with a cooling device 17′. Precipitation reactionsalready begin in the disintegration and spraying apparatus, as the gascomes in contact with the calcium hydroxide sludge or other mineralsubstance.

When treated in the disintegration and spraying apparatus 14, thecalcium hydroxide sludge forms very fine drops and particles which willbe dispersed from the apparatus 14 to a section 34′ of the gas spacesurrounding the apparatus. Fine drops and particles are hurled out ofthe apparatus 14, mainly from its outer ring area, as a mist-like flow36. Precipitation reactions outside the feed apparatus may continue fora relatively long time as fine drops and particles disperse widely inthe precipitation vessel 12. Precipitating calcium carbonate andpossibly some unprecipitated calcium hydroxide lands on the bottom ofthe precipitation vessel and is discharged from the vessel through thepipe 20.

The size, shape, width and height of the precipitation vessel 12 can beselected such that the drops and particles which are hurled out of thefeed apparatus remain in the gas space 34′ of the precipitation vesselfor a dwell time which is as appropriate as possible. For example,increasing the height of the precipitation reactor 12, making ittower-like, or increasing its diameter, increases the dwell time of thecalcium hydroxide sludge.

Processes in the precipitation reactor 10 may also be regulated, forexample, by adjusting the number of rings, the distance between therings, the distance between the blades on each ring and the dimensionand position of the blades on the disintegration and spraying apparatus14.

FIGS. 3 and 4, which illustrate a second precipitation reactor accordingto the invention, with its disintegration and spraying apparatus 14, usethe same reference numbers as presented in FIGS. 1 and 2 whereapplicable. The second precipitation reactor 10, presented in FIG. 3,according to the invention, differs from the apparatus presented inFIGS. 1 and 2, chiefly such that the reactor comprises a disintegrationand spraying apparatus 14 equipped with a closed outer ring 32, which,at the same time, forms the whole precipitation reactor. Theprecipitation reactor does not include a separate precipitation areaextending outside the disintegration and spraying apparatus. Thesolution presented in FIGS. 3 and 4 is suitable for use, for example,when the precipitation reactions may be assumed to have already beencompleted as desired in the gas space of the disintegration and sprayingapparatus, or if there are several reactors.

In the disintegration and spraying apparatus presented in FIGS. 3 and 4,the outermost ring 28″ is surrounded by a housing 40 which closes thering. The housing comprises a discharge opening 42 for discharging theprecipitated calcium carbonate from the apparatus 14. The precipitatedcalcium carbonate may be directed from the discharge opening 42 forfurther treatment or processing, or it can be circulated through thepipe 43, into the pipe 16, and back to the disintegration and sprayingapparatus 14. The pipe 16 may be equipped with a cooler 45.

Two or more of both types of precipitation reactors presented in FIGS. 1and 3 can be arranged in a sequential series. FIG. 5 illustrates a groupof three precipitation reactors of the type presented in FIG. 1. Thereference numbers are the same as in the previous figures whereapplicable.

FIG. 5 illustrates three precipitation reactors 10, 10′ and 10″ wherecalcium hydroxide is brought into contact with carbon dioxide gas inorder to carbonate the calcium hydroxide into calcium carbonate, i.e. toprecipitate CaCO₃. The reactors are connected sequentially, such thatthe suspension of the precipitated carbonate and the unprecipitatedcalcium hydroxide is directed from the discharge pipe 20 of the firstreactor 10 to the feed pipe 16′ of the second reactor 10′.Correspondingly, the suspension containing a larger amount of carbonatedcalcium carbonate is directed through the discharge pipe 20′ of thesecond reactor 10′ into the feed pipe 16″ of the third reactor 10″.

The gas containing carbon dioxide is led to each reactor through thepipes 18, 18′, 18″. The gas containing carbon dioxide is led through thefeed pipe 18 to the first reactor 10, which induces precipitation(carbonation) and the formation of a carbonate in the disintegration andspraying apparatus 14. The same gas, or other gas containing carbondioxide, can be led to the second and third precipitation reactors 10′,10″ through the pipes 18′, 18″ to complete the precipitation reactions(carbonation). The gas is removed from the reactors through dischargepipes 21, 21′, 21″.

According to the invention, in the case presented in FIG. 5, coolers 17and 17′ are fitted to the discharge pipes 20 and 20′ to cool thematerial to be fed into the precipitation reactors 20′ and 20″.

Break or storage tanks can be fitted between the precipitation reactors10, 10′ and 10″, where the product containing calcium carbonate comingfrom the previous precipitation reactor can be stored for a while, forsome minutes, or even for a longer time.

FIG. 6 illustrates a second precipitation reactor group, which comprisesone sequentially fitted precipitation reactor 10 of the type presentedin FIG. 3, and two precipitation reactors 10′, 10″ of the type presentedin FIG. 1 which are fitted to the series.

Calcium hydroxide is led through the pipe 16, and gas containing carbondioxide is led through the pipe 18 to the first precipitation reactor10, i.e. to the disintegration and spraying apparatus 44. Materialexiting from the precipitation reactor 10 is directed into a gasseparator 50, where the gas containing carbon dioxide is separated fromthe material containing calcium hydroxide and calcium carbonate. The gascontaining carbon dioxide and vapour is directed through the pipe 54into a washing and cooling apparatus 52, from where the gas containingcarbon dioxide is directed through the pipe 18′ to the disintegrationand spraying apparatus of the second precipitation reactor 10′. Thematerial containing calcium hydroxide and calcium carbonate is directedfrom the gas separator through the pipe 16′, which is equipped with acooler 11, to the disintegration and spraying apparatus 14 of the secondprecipitation reactor.

The gas, typically containing vapour and carbon dioxide, is removed fromthe top section of the precipitation reactor 10′ through the pipe 21.The gas is led for treatment in the gas washing and cooling apparatus52. In the apparatus 52, some of the treated gas containing carbondioxide is circulated through the pipe 18′ back to the precipitationreactor 10′, and the rest 18″ is directed to the next precipitationreactor 10″. The precipitated calcium carbonate and the unprecipitatedcalcium hydroxide, gathering at the bottom section of the precipitationreactor, is discharged to the discharge pipe 20.

The third precipitation reactor 10″, as presented in FIG. 6, operatesmainly on the same principle as the second precipitation reactor 10′.The material, which has been removed from the bottom of the secondreactor 10′ to the pipe 20, and which contains calcium hydroxide inaddition to the precipitated calcium carbonate, is directed through thepipe 16″ along the bottom to the disintegration and spraying apparatus14′ of the third reactor 10″. From the washing and cooling apparatus 52,the gas containing carbon dioxide is directed to the third reactor 10″through the pipe 18″. Completely precipitated calcium carbonate isdischarged from the bottom of the third reactor 10″ through the pipe20′. The gas is removed from the top section of the third reactor 10″through pipe 21′ and is led to the washing and cooling apparatus 52 forfurther circulation.

FIG. 7 illustrates a precipitation reactor group comprising oneprecipitation reactor 10 of the type presented in FIG. 3 and threeprecipitation reactors 10′, 10″, 10′″ of the type presented in FIG. 1which have been fitted to the series. The first precipitation reactoroperates as the precipitation reactor presented in FIG. 6. The threeprecipitation reactors 10′, 10″, 10′″, as presented in FIG. 1, arefitted on top of each other and the calcium hydroxide sludge is fed fromthe top to the feed apparatuses 14, 14′, 14″ located in theprecipitation reactors. The second reactor 10′ is on the top and thethird reactor 10′″ is on the bottom, whereupon the material containingthe calcium hydroxide and the calcium carbonate flows mostly downwardswhen travelling through the reactors.

EXAMPLES

The purpose of the experiments presented in the following examples is topresent how it is possible to affect to the specific surface area andparticle size of the precipitated calcium carbonate by applying asolution according to the invention. The purpose is only to illustrate,not to limit the invention.

The results of the experiments KP1-KP4 are presented in the followingsection. In these experiments, according to the invention, precipitationwas carried out on the calcium hydroxide sludge using a gas containingcarbon dioxide. In experiments KP1 and KP2, the calcium hydroxide sludgewas cooled to 13° C. before initiation of carbonation. Moreover, inexperiment KP2, the sludge to be carbonated was cooled duringcarbonation. In experiments KP3 and KP4, the initial temperature of thecalcium hydroxide sludge was 30° C. In experiment KP4, sorbitol wasadded to the calcium hydroxide sludge.

In all experiments, the dry matter content, quality of the Ca(OH)₂ andthe composition of the gas containing CO₂ were the same.

The dry matter content of the Ca(OH)₂ sludge was adjusted so that thedry matter of the final product, the precipitated calcium carbonate, was17%.

KP1. The temperature of the Ca(OH)₂ sludge was adjusted to 13° C., afterwhich the sludge was pumped through the precipitation reactor. Theexcess amount of gas containing CO₂ was fed into the apparatus. As theCa(OH)₂ sludge was being fed into the precipitation reactor, it formedvery fine mist-like drops which were mixed with the gas containing CO₂.This treatment was repeated three times, after which the pH of theobtained precipitated calcium carbonate, PCC-sludge, was 6.8. During thetreatment, the temperature rose to 55° C.

KP2. The second experiment was conducted as the first experiment, exceptthat the sludge was cooled during the process, so that its temperaturedid not exceed 27° C. After the third treatment, the pH of the PCCsludge was 6.9.

KP3. The third experiment was conducted as the first experiment, exceptthat the temperature of the Ca(OH)₂ sludge was 30° C. at the beginningof the experiment.

During the treatment, the temperature of the sludge rose to 61° C. Afterthe treatment, the pH of the sludge was 6.8.

KP4. The fourth experiment was conducted as the third experiment, exceptthat 1.5% sorbitol, as calculated from the PCC to be produced, was addedto the Ca(OH)₂ sludge. During this treatment the temperature of thesludge rose to 60° C. After the third treatment, the pH of the sludgewas 6.9.

A specific surface area was measured (using a Micrometrics Flowsorb IIIdevice) from the samples thus produced, and the average size of theparticles was determined using electron microscope images. The resultsare presented in Table 1.

TABLE 1 Experiment KP1 KP2 KP3 KP4 cooling — yes — — initial temperature13 13 30 30 ° C. max. temperature 55 27 61 60 ° C. sorbitol — — — 1.5 %d₅₀ 40 30 50 35 nm specific surface area 60 81 48 65 m²/g

Experiments KP1 and KP2 demonstrate that by cooling the sludge (Ca(OH)₂)being carbonated during the precipitation, it was possible to obtain acalcium carbonate product, in which the particle size was remarkablysmaller (30) than without cooling (40). Correspondingly, the specificsurface area of the product in KP2 (81) was larger than in experimentKP1 (60).

Experiments KP1 and KP3 demonstrate that in KP1, in which the initialtemperature of the Ca(OH)₂ sludge was lowered, it is possible to obtaina calcium carbonate product in which the specific surface area is largerand the particle size smaller than in KP3.

Experiments KP3 and KP4 show that it is possible to obtain smallerparticles and a correspondingly larger specific surface area whensorbitol is added.

The solution according to the invention has achieved a simple,continuously operating and fast technical-economical method andapparatus for industrially producing a calcium carbonate product formedof very small, <100 nm, and mostly separate particles. The methodenables the complete carbonation of calcium hydroxide easily. Thereaction time is short, i.e. carbonation is fast, which leads to theformation of small, homogenous particles with a large specific surfacearea. The process is less dependent on the particle size and dry mattercontent of the calcium hydroxide input.

The method and apparatus are simple. The main components of the productcan typically be added to the process simultaneously, in which case thesubstances react with each other immediately.

The method and apparatus also lead to an economical final result. Theapparatus used is energy efficient with regard to the obtained results.The apparatus converts the fed mineral substance, sludge or a solutioncontaining calcium hydroxide, into very fine drops or particles. In themethod, the source material can be calcium hydroxide with a high drymatter content, which also produces a calcium carbonate product with ahigh dry matter content.

The method allows for a wide variety of variables, which makes it easierto find the correct adjustments for each specific case. Cooling makes itpossible to create an even processing temperature if necessary.

Using a polyol such as sorbitol as an additive in association with thecarbonation process, it is possible to influence the specific surfacearea, particle size and/or homogeneity of the calcium carbonate to beformed. In that case, not much cooling is needed, or possibly none atall, for producing the desired calcium carbonate product with a verysmall particle size. On the other hand, even smaller particles sizes canbe obtained using cooling and a polyol addition at the same time.

Also, other additives which directly affect the surface chemicalproperties of the product can be used.

The invention is not intended to be limited to the examples presentedabove; on the contrary, it is intended that the invention be broadlyadapted within the scope of the claims presented below.

1. An apparatus for producing a calcium carbonate product from calciumhydroxide, comprising small calcium carbonate particles, which apparatuscomprises at least one precipitation reactor into which the calciumhydroxide or other corresponding Ca⁺⁺ ion source is disintegrated orsprayed as fine drops and/or particles and inside of which the calciumhydroxide is brought into contact with gas which contains carbon dioxideand apparatuses for feeding the calcium hydroxide or other correspondingCa⁺⁺ ion source into the precipitation reactor, and apparatuses forremoving the calcium carbonate product from the precipitation reactor,wherein the apparatus additionally comprises a disintegration andspraying apparatus, operating on the principle of a pin mill, fordisintegrating the calcium hydroxide as small drops and/or particlesinto the precipitation reactor, wherein the disintegration and sprayingapparatus is located inside the precipitation reactor, and coolingapparatuses for maintaining a lowered temperature in the precipitationreactor.
 2. The apparatus according to claim 1, wherein the coolingapparatuses comprise apparatuses for lowering the temperature of amaterial, such as the calcium hydroxide, carbon dioxide or circulatingcalcium carbonate to be fed into the precipitation reactor and/orapparatuses for keeping the temperature of the material in theprecipitation reactor low.
 3. The apparatus according to claim 1,wherein the apparatus comprises two or several sequential precipitationreactors which have flow connections to each other, and wherein coolersfor cooling the calcium carbonate material flowing from one reactor toanother are arranged between the sequentially arranged precipitationreactors.
 4. The apparatus according to claim 1, wherein the ring speedof the rotors of the disintegration and spraying apparatus operating onthe principle of a pin mill is 1-250 m/s, and wherein the speeddifference of the rings of the rotors rotating in different or in thesame directions, or the speed difference of the rings of adjacent rotorsand stators is 5-400 m/s.
 5. The apparatus according to claim 4, whereinthe speed difference of the rings of the rotors rotating in different orin the same directions, or the speed difference of the rings of adjacentrotors and stators, is 5-200 m/s.